Page 2. The mesopelagic zone

Because the mesopelagic zone (200–1,000 metres deep) and
those beneath it provide similar habitats worldwide, some of
the same species are found in both hemispheres. About 20% of
the food made in the epipelagic zone makes its way down to
the mesopelagic. Conversely, at night many mesopelagic
organisms migrate upwards into the darkened epipelagic zone.
This gives the mesopelagic creatures the best of both worlds.
They can feed at night in the rich waters of the epipelagic
zone and hide during the day in the dark, cold,
oxygen-depleted waters of the mesopelagic, safe from most
predators.

A false sea floor

When sonar became widely used during the Second World War,
operators detected an apparent sea floor at a depth of
300–500 metres during the day, but nearer the surface at
night. This proved to consist of millions of organisms,
including fish and zooplankton, which move up through the
water at dusk to feed on microscopic plants growing where the
sun penetrates. On moonlit nights this layer remains deeper,
although the animals within it may respond to clouds passing
over the moon. Many species including jellyfish, squid and
crustaceans are involved in this vertical migration, but it
was mainly fish that gave rise to the echo picked up by the
sonar operators. The sound was reflected off their swim
bladders – gas bags that enable them to fine-tune their depth
without expending a lot of energy. The phenomenon of the
false sea floor became known as the deep scattering layer or
DSL, because it scatters the sonar signal.

Fish adaptations to the zones

Typically, species such as lanternfish (Myctophids) and
bristlemouths (Gonostomatids), which make vertical migrations
each day, possess a swim bladder. They also have
well-developed muscles and bones, and are usually somewhat
streamlined. By contrast, fish that remain as ‘sit and wait’
predators in the mesopelagic zone lack a swim bladder, and
have flabby muscles and watery flesh. Because they do not go
anywhere, streamlining has either not evolved or has
disappeared.

Bathypelagic fish (at depths of 1,000–4,000 metres) are
unable to undertake vertical daily migrations because below a
certain depth the pressure is too great and the swim bladder
is unable to function – gas cannot be produced. Most of these
fish lack swim bladders or, like orange roughy, have bladders
that are filled with wax. Sharks do not have swim bladders
but are able to move vertically. To plane up and down they
rely on strong swimming and the buoyancy provided by their
oily livers – oil is less dense than water.

Light-producing creatures

Many mesopelagic animals possess bioluminescent
(light-producing) organs known as photophores. In some
species the light is produced by specialised tissues, in
others by symbiotic bacteria. Many mesopelagic fish, shrimps
and squid have photophores on their undersides that match
surface brightness. They reduce the light output during
cloudy days, and boost it when it is sunny. The photophores
may be turned off at night to make the animal relatively
invisible.

Bioluminescence may play other roles, as there are often
sexual differences in photophore distribution. In some
species the typically blue-green light may indicate the
species or sex of an animal, while in others the light may
attract prey or enable the predator to see its quarry. Faced
with a predator, some species of shrimp and squid eject
clouds of luminescent ink to produce a replica of themselves,
and then swim away while the predator attacks it.

Glow in the dark

Bioluminescence (light produced by living things)
happens mostly in the ocean. While a few land animals such
as glow-worms use it, it is almost unknown in fresh water.
It is a cold light produced by a chemical reaction inside
an organism. The loosejaw fish Malacosteus niger
can produce a red searchlight to spot its prey while it
remains unseen.

Vision in the mesopelagic zone

Typically, mesopelagic fish have large eyes that gather
ambient light. Some have tubular eyes that are upward looking
and give sharp forward but poor sideways vision. A secondary
retina extends up the sides of the tube and dramatically
increases the field of view. Parallel evolution has produced
similar optical solutions in at least one octopus
(Amphitretus) and some krill
(Stylocheiron). Some tubular-eyed fish have a yellow
filter that enables them to distinguish between natural light
and bioluminescence, thereby defeating the
counter-illumination systems used by their prey.

Other mesopelagic organisms

Other mesopelagic organisms include copepods and krill
(planktonic animals) that eat the faeces of epipelagic
copepods and any other matter that reaches that depth. Many
krill and shrimp possess light organs. Ostracods, amphipods,
arrow worms (Chaetognaths), jellyfish, siphonophores, comb
jellies, larvaceans and pteropods are also common. Squid,
including juveniles of the giant squid Architeuthis
dux and the considerably larger colossal squid, are also
found here.